Semiconductor manufacturing, such as the fabrication of integrated circuits, typically entails the use of photolithography. A semiconductor substrate on which circuits are being formed, usually a silicon wafer, is coated with a material, such as a photoresist, that changes solubility when exposed to radiation. A lithography tool, such as a mask or reticle, positioned between the radiation source and the semiconductor substrate casts a shadow to control which areas of the substrate are exposed to the radiation. After the exposure, the photoresist is removed from either the exposed or the unexposed areas, leaving a patterned layer of photoresist on the wafer that protects parts of the wafer during a subsequent etching or diffusion process.
The photolithography process allows multiple integrated circuit devices or electromechanical devices, often referred to as “chips,” to be formed on each wafer. The wafer is then cut up into individual dies, each including a single integrated circuit device or electromechanical device. Ultimately, these dies are subjected to additional operations and packaged into individual integrated circuit chips or electromechanical devices.
During the manufacturing process, variations in exposure and focus require that the patterns developed by lithographic processes be continually monitored or measured to determine if the dimensions of the patterns are within acceptable ranges. The importance of such monitoring, often referred to as process control, increases considerably as pattern sizes become smaller, especially as minimum feature sizes approach the limits of resolution available by the lithographic process. In order to achieve ever-higher device density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting metallization lines, spacing and diameter of contact holes and vias, and the surface geometry such as corners and edges of various features. Features on the wafer are three-dimensional structures and a complete characterization must describe not just a surface dimension, such as the top width of a line or trench, but a complete three-dimensional profile of the feature. Process engineers must be able to accurately measure the critical dimensions (CD) of such surface features to fine tune the fabrication process and to assure a desired device geometry.
Typically, CD measurements are made using instruments such as a scanning electron microscope (SEM). In a scanning electron microscope (SEM), a primary electron beam is focused to a fine spot that scans the surface to be observed. Secondary electrons are emitted from the surface as it is impacted by the primary beam. The secondary electrons are detected, and an image is formed, with the brightness at each point of the image being determined by the number of secondary electrons detected when the beam impacts a corresponding spot on the surface. As features continue to get smaller and smaller, however, there comes a point where the features to be measured are too small for the resolution provided by an ordinary SEM.
Transmission electron microscopes (TEMs) allow observers to see extremely small features, on the order of nanometers. In contrast to SEMs, which only image the surface of a material, TEMs allows the additional capability to analyze the internal structure of a sample. In a TEM, a broad beam impacts the sample and electrons that are transmitted through the sample are focused to form an image of the sample. The sample must be sufficiently thin to allow many of the electrons in the primary beam to travel though the sample and exit on the opposite site. Samples are typically less than 100 nm thick.
In a scanning transmission electron microscope (STEM), a primary electron beam is focused to a fine spot, and the spot is scanned across the sample surface. Electrons that are transmitted through the substrate are collected by an electron detector on the far side of the sample, and the intensity of each point on the image corresponds to the number of electrons collected as the primary beam impacts a corresponding point on the surface.
As semiconductor geometries continue to shrink, manufacturers increasingly rely on transmission electron microscopes (TEMs) for monitoring the process, analyzing defects, and investigating interface layer morphology. The term “TEM” as used herein refers to a TEM or a STEM, and references to preparing a sample for a TEM are to be understood to also include preparing a sample for viewing on an STEM. Because a sample must be very thin for viewing with transmission electron microscopy (whether TEM or STEM), preparation of the sample can be delicate, time-consuming work.
Thin TEM samples cut from a bulk sample material are known as “lamellae”. Lamellae are typically less than 100 nm thick, but for some applications a lamella must be considerably thinner. With advanced semiconductor fabrication processes at 30 nm and below, a lamella needs to be less than 20 nm in thickness in order to avoid overlap among small scale structures. Currently, thinning below 30 nm is difficult and not robust. Thickness variations in the sample result in lamella bending, overmilling, or other catastrophic defects. For such thin samples, lamella preparation is a critical step in TEM analysis that significantly determines the quality of structural characterization and analysis of the smallest and most critical structures.
Several techniques are known for preparing TEM specimens. These techniques may involve cleaving, chemical polishing, mechanical polishing, or broad beam low energy ion milling, or combining one or more of the above. The disadvantage to these techniques is that they are not site-specific and often require that the starting material be sectioned into smaller and smaller pieces, thereby destroying much of the original sample.
Other techniques generally referred to as “lift-out” techniques use focused ion beams to cut the sample from a substrate or bulk sample without destroying or damaging surrounding parts of the substrate. Such techniques are useful in analyzing the results of processes used in the fabrication of integrated circuits, as well as materials general to the physical or biological sciences. These techniques can be used to analyze samples in any orientation (e.g., either in cross-section or in plan view). Some techniques extract a sample sufficiently thin for use directly in a TEM; other techniques extract a “chunk” or large sample that requires additional thinning before observation. In addition, these “lift-out” specimens may also be directly analyzed by other analytical tools, other than TEM. Techniques where the sample is extracted from the substrate within the focused ion beam (“FIB”) system vacuum chamber are commonly referred to as “in-situ” techniques; sample removal outside the vacuum chamber (as when the entire wafer is transferred to another tool for sample removal) are call “ex-situ” techniques.
Samples which are sufficiently thinned prior to extraction are often transferred to and mounted on a metallic grid covered with a thin electron transparent film for viewing. FIG. 1A shows a sample mounted onto a prior art TEM grid 10. A typical TEM grid 10 is made of copper, nickel, or gold. Although dimensions can vary, a typical grid might have, for example, a diameter of 3.05 mm and have a middle portion 12 consisting of cells 14 of size 90×90 μm2 and bars 17 with a width of 35 μm. The electrons in an impinging electron beam will be able to pass through the cells 14, but will be blocked by the bars 17. The middle portion 12 is surrounded by an edge portion 16. The width of the edge portion is 0.225 mm. The edge portion 16 has no cells, with the exception of the orientation mark 18. The thickness 15 of the thin electron transparent support film is uniform across the entire sample carrier, with a value of approximately 20 nm. A thin carbon film 19 is attached to the bottom side of TEM grid 10. TEM specimens to be analyzed are placed or mounted within cells 14 on top of carbon film 19.
In one commonly used ex-situ sample preparation technique, a protective layer 22 of a material such as tungsten is deposited over the area of interest on a sample surface 21 as shown in FIG. 2 using electron beam or ion beam deposition. Next, as shown in FIGS. 3-4, a focused ion beam using a high beam current with a correspondingly large beam size is used to mill large amounts of material away from the front and back portion of the region of interest. The remaining material between the two milled rectangular trenches 24 and 25 form a thin vertical sample section 20 that includes an area of interest. The angle of the FIB (not shown) used in the milling is generally angled at 90° from the sample surface 21. This allows for the FIB to mill straight down. The trench 25 milled on the back side of the region of interest is smaller than the front trench 24. The smaller back trench is primarily to save time, but the smaller trench also prevents the finished sample from falling over flat into larger milled trenches which may make it difficult to remove the specimen during the micromanipulation operation. When sample section 20 is eventually extracted, it will lay horizontally on a TEM/STEM machine exposing a TEM normal viewing side 23.
As shown in FIG. 5, once the specimen reaches the desired thickness, the stage is tilted and a U-shaped cut 26 is made at an angle partially along the perimeter of the sample section 20, leaving the sample hanging by tabs 28 at either side at the top of the sample. The sample section 20 that is cut out has a TEM normal viewing side 23 in the shape of a rectangle. The small tabs 28 allow the least amount of material to be milled free after the sample is completely FIB polished, reducing the possibility of redeposition artifacts accumulating on the thin specimen. The sample section is then further thinned using progressively finer beam sizes. Finally, the tabs 28 are cut to completely free the thinned lamella 27. When the lamella 27 is cut out and placed horizontally—The lamella 27 is generally a rectangular shape. Once the final tabs of material are cut free lamella 27 may be observed to move or fall over slightly in the trench.
In ex-situ processes, the wafer containing lamella 27 is removed from the vacuum chamber containing the FIB and placed under an optical microscope equipped with a micromanipulator. A probe attached to the micromanipulator is positioned over the lamella and carefully lowered to contact it. Electrostatic forces will attract lamella 27 to the probe tip 28 (shown in FIG. 6) or the micromanipulator can have a hollow center wherein it can create a vacuum through the probe tip to secure the lamella. The tip 28 with attached lamella 27 is then typically moved to a TEM grid 10 as shown in FIG. 7 and lowered until lamella 27 is placed on the grid in one of the cells 14 between bars 17. FIG. 8 is a picture of a lamella 27 on a traditional carbon grid. As shown in FIG. 8, even with the successful transportation of lamella 27 onto the carbon film 19, the orientation of the lamella 27 is difficult to determine. On account of the general rectangular shape of the lamella, it is difficult to determine whether the lamella 27 has turned 180° or has been inverted during the process of moving the lamella 27 from the vacuum chamber to the carbon grid 13.
The use of traditional carbon grid 13 poses a technical problem. The carbon grid 13 includes a carbon film 19 that can impact chemical analysis, such as EDS or EELS. Electron energy loss spectroscopy (EELS) is a form of spectroscopy wherein a material is exposed to a beam of electrons with a known, narrow range of kinetic energies. By measuring the amount of energy loss, the spectroscopy can determine the types of atoms, and the numbers of atoms of each type in a given sample. EELS is a compliment to energy-dispersive x-ray spectroscopy (variously called EDX, EDS, XEDS, etc.), which is another common spectroscopy technique available on many electron microscopes. EDX has the capability to identify the atomic composition of a material that is sensitive to heavier elements. In order to reduce the impact of the carbon background in chemical analysis, various analytical techniques will subtract the background interference of the carbon film.
Another typical procedure for studying lamella uses in-situ analysis. A common in-situ extraction technique is described in U.S. Pat. No. 6,570,170 to Moore, which describes extracting out a sample by making a “U”-shaped cut and then cutting the sample at an angle from the missing side of the “U” to undercut and free the sample. After the sample is freed, a probe 42 is attached to the sample by FIB-induced chemical vapor deposition and it is lifted out. This process typically results in a chunk-type sample 30, which is generally wedge shaped and approximately 10×5×5 μm in size. This sequence of steps is illustrated in FIG. 9 and FIG. 10.
In-situ and ex-situ methods of TEM sample preparation have their own advantages, but they also have their own shortcomings. In in-situ methods, each of the lamella is cut, attached to the microprobe, and attached to the sample holder individually. The individual manipulation of the lamella provides a high degree of confidence in knowing the orientation of the lamella. But these methods are typically very time-consuming and labor intensive. It further requires the sample to be welded or affixed to a TEM grid inside a vacuum chamber, which requires a large amount of valuable FIB time. In addition, because the process is performed over carbon films, analytical techniques are required to subtract the carbon background information by sampling the carbon film areas outside the lamella. In addition, CD metrology often requires multiple samples from different locations on a wafer to sufficiently characterize and qualify a specific process. In some circumstances, for example, it will be desirable to analyze from 15 to 50 TEM samples from a given wafer. When so many samples must be extracted and measured using known methods, the total time to process the samples from one wafer can be days or even weeks. Even though the information that can be discovered by TEM analysis can be very valuable, the entire process of creating and measuring TEM samples has historically been so labor intensive and time consuming that it has not been practical to use this type of analysis for manufacturing process control.
Although ex-situ methods do not require the labor intensive and time consuming manipulation inside the vacuum chamber, they are unreliable and require a great deal of operator experience. Even with experienced operators, the success range is only about 90%. It can be time consuming and difficult to locate a lamella site and the extraction probe must be very carefully moved into position to avoid damaging the sample or the probe tip. Once a lamella has been completely freed, it can move in unpredictable ways; it can fall over in the trench or in some cases it can actually be pushed up and out of the trench by electrostatic forces. This movement can make it difficult to locate and/or pick up the lamella with the extraction probe. The electrostatic attraction between the probe and the sample is also somewhat unpredictable. In some cases, the lamella may not stay on the probe tip. Instead, it can jump to a different part of the probe. In other cases, the lamella may fall off while the sample is being moved. If the lamella is successfully transferred to the TEM grid, it can be difficult to get the lamella to adhere to the grid support film rather than the probe tip. The lamella will often cling to the probe tip and must be essentially wiped off onto the film. As a result, it is difficult to control the precise placement or orientation of the lamella when it is transferred to the TEM grid. The lamella typically has a region of interest that is intended for imaging. If the lamella 27 is close to bars 17, it is often difficult to determine if the region of interest is properly placed over the carbon grid and if the region of interest is properly aligned with the holes in the carbon film.
Some carbon films that are used with TEM lamella analysis are carbon grids with holes. Carbon grids with holes, such as the Quantifoil™ 2:1 grids provide the ability to position a region of interest over the holes such that there is less optical and spectral interference with the carbon. A typical carbon grid of this characteristics will have a hole size of 2 μm. Unfortunately, the region of interest in the lamella often exceed 2 μm in length, and the typical carbon grids with holes are not able to fully contain the region of interest. Even with superior TEM systems, the optical chain and mechanical motion control has at least 1 μm of error during placement. This results in a failure to properly place the region of interest in nearly 80% of the cases.
Experienced ex-situ plucking users can use a standard glass rod micro manipulator to move and orient the lamella 27 based on optical imaging systems, but any unforeseen motion on the lamella 27 during the plucking and placing process eliminates any confidence of orientation. Unforeseen motion during the process occurs approximate 25% of the time. In addition, the ability to set the sample into a very specific region of interest has a large amount of uncertainty.
What is needed is an improved method for TEM sample analysis, including an ability to create samples that allow for easy orientation of the lamella so that its region of interest can be properly imaged with little to no interference from the carbon film during chemical analysis.